Prenatally detected abdominal and pelvic masses are commonly cystic in morphology and usually seen at mid-trimester sonography. Sonography is the favored imaging modality for postnatal evaluation of these lesions in newborns, given the availability, low cost, lack of ionizing radiation, lack of sedation, and high spatial resolution in small patients. The differential diagnosis of abdominopelvic cystic masses in the newborn is broad given that they can arise from many organs and may have overlapping features on imaging. This article illustrates an approach to the postnatal sonographic evaluation of prenatally detected cystic abdominal and pelvic masses based on their anatomic location as well as distinctive sonographic characteristics, which can aid in accurate diagnosis and guide appropriate management.